One Fat Cat Other Losing Weight

Both my cats had gained way to much weight so we switched them to a special diet food and are only feeding them once a day.My older cat Talon is losing weight no problem but our younger cat Cali seems to just be gaining more and more weight.She is very petite in size but she is at 16 lbs.The whole situation makes no sense to me Talon is very lazy and does nothing all day.Cali is very active playing all the time.What can we do to help her loose this weight?